Single-step Selective Oxidation of Methane to Methanol in the Aqueous Phase on Iridium-based Catalysts

Abstract In this work, one-step conversion of methane to methanol using molecular oxygen as the oxidant in the presence of CO in aqueous solutions on copper or palladium promoted Ir-ZSM-5 catalyst is first reported. The addition of a second metal to Ir-ZSM-5 promotes the catalyst activity, while product selectivity can be tuned either to methanol on IrCu-ZSM-5 or to formic acid exclusively on IrPd-ZSM-5. Most effective is the combination of the three metal species together. Approximately 1200 µmol/gcat methanol, or ∼23.4 mol of methanol per mol of Ir, are formed on the IrCuPd trimetallic system (methanol selectivity ∼80%) at 150 °C in 1 h. Our results also demonstrate that atomically dispersed Ir(I)(CO)2 species formed in the presence of CO can activate the C-H bond of methane to methyl species at temperatures below 150 °C. The good stability in cyclic operation is an additional attribute, rendering this type of catalyst a “front-runner” in future catalyst development for direct methane-to-liquid oxygenates.